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Leaves as Large as Elephant Ears

Posted: June 25, 2015

Sun leaves versus shade leaves: location and leaf physiology.

“Those leaves are huge! Are you fertilizing those trees?” exclaimed a participant on a recent walking tour of a timber harvesting demonstration site in central Pennsylvania. Immediately, I had a mental flashback to summers in Pittsburgh as a child.

One of the outside games I had fun playing was “Find the biggest leaf.” It was not a game that most of the kids in the neighborhood played. Really there were only two of us who found joy in finding the biggest leaves. Through this game, we learned something about tree and plant biology. First, we discovered that, as youngsters, we were ideally structured for finding big leaves – we were closer to the ground. We soon learned that big leaves were more common down low on trees and other plants. Second, we noticed the big leaves weren’t common on the sycamores lining the streets; rather, they were in the shaded alleys and along the edges of unkempt properties, the abandoned old farm field and barn, and near the strip mine where the young trees were at a convenient height.

We’d call the giant leaves “elephant ears.” The game had one major rule –you had to stay with the same species. Sycamore leaves competed with sycamore leaves, Norway maple leaves with Norway maple leaves, forsythia leaves with forsythia leaves. You get the picture. Likely we never found a leaf as large as even a small elephant’s ear, but we learned and searched. It was fun!

So, why are some leaves-even on the same individual tree or plant-bigger than others? Biologists describe the size difference simply as “sun leaves” and “shade leaves.” For the most part, leaves growing in the sun are physiologically different from those growing in the shade. The difference relates to the environment they have to deal with and the light resources they receive.

At the top of a tree, a leaf receives more light and wind. These sun leaves are thicker and smaller. They are thicker because of the distribution of chloroplasts within the palisade cells, which are tall cells standing on end just under the leaves’ “skin.” This arrangement makes the leaves efficient at converting light to make sugars. It also suggests upper leaves are often darker green. Because upper leaves gather more light, they get hotter. Therefore, they have more and smaller stomata, which are a leaf structure designed to exhaust oxygen and water – a cooling mechanism – than are found on shade leaves. The advantage of having lots of small stomata on a smaller leaf is the ability to adjust moisture loss not only to heat, but to increased wind moving across the leaf’s surface.

Lower down in the tree, where the leaves are shaded by those higher up, the logic begins to reverse. The leaf is thinner because the palisade cells are shorter. This arrangement makes sense because light is less intense and does not reach as deeply into the leaf, and the leaf is often a lighter shade of green. Because the chloroplasts are closer to the surface, the leaf area increases to make it more efficient resulting in bigger leaves. The stomata are larger and wider spaced. In general the shade leaf stays cooler, has less wind moving across its surface, and needs less cooling.

What was a fun game for us as children turned out to be a great lesson: Trees and other plants respond to their environment in ways that increase their efficiency. Sometimes the reasons behind the differences in plant structures seem obscure, but nature is about efficiency. It is fun to explore our forest environment and to share our observations and thoughts. If you have the chance this summer, go looking for an “elephant ear” in your woodlot. While you are at it, invite a child to enjoy the game, “Who can find the biggest leaf?”

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. For a list of free publications, call 800-235-9473 (toll free), send an email to RNRext@psu.edu, or write to Forest Stewardship Program, Natural Resources Extension, The Pennsylvania State University, 416 Forest Resources Building, University Park, PA 16802. The Pennsylvania DCNR Bureau of Forestry, USDA Forest Service, and Center for Private Forests at Penn State in Partnership with Penn State’s Department of Ecosystem Science and Management, sponsor the Forest Stewardship Program in Pennsylvania.